Cationic polymerization of 2,4,6,8-tetramethylcyclotetrasiloxane processed by tuning the pH of the miniemulsion

The preparation of 2,4,6,8-tetramethylcyclotetrasiloxane (D4H) miniemulsions stabilized by commercial surfactants, i.e. sodium dodecylbenzenesulfonate (NaDBSA) and sodium laurate/lauric acid mixture, was studied and is first reported. Then, the pH was tuned in order to process polymerization through interfacial protonic initiation. With lauric acid, no polymerization was observed, even at high temperature or large acid contents. Such effect is ascribed to the poor dissociation and reactivity of high pKa's carboxylic acids. With NaDBSA, acidification by HCl was enough to initiate the polymerization, but the pH of the continuous phase was shown to have a strong effect on the polymerization process. Optimal conditions (at pH 5) gave chains with average number molar masses around 16,000 g mol−1 and polydispersity index close to 1.6, for a total conversion in polymer of about 60% after 2 h reaction. Lower pH (4.2) quickly led to a cross-linked network whereas at higher pH (6.1), conversions were too slow (around 10% after 350 min).